3 <title>Controlling the Embedded VM</title>
4 <link rel=stylesheet href="android.css">
8 <h1>Controlling the Embedded VM</h1>
11 <li><a href="#introduction">Introduction</a> (read this first!)
12 <li><a href="#checkjni">Extended JNI Checks</a>
13 <li><a href="#assertions">Assertions</a>
14 <li><a href="#verifier">Bytecode Verification and Optimization</a>
15 <li><a href="#execmode">Execution Mode</a>
16 <li><a href="#dp">Deadlock Prediction</a>
17 <li><a href="#stackdump">Stack Dumps</a>
18 <li><a href="#dexcheck">DEX File Checksums</a>
19 <li><a href="#general">General Flags</a>
22 <h2><a name="introduction">Introduction (read this first!)</a></h2>
24 <p>The Dalvik VM supports a variety of command-line arguments
25 (use <code>adb shell dalvikvm -help</code> to get a summary), but
26 it's not possible to pass arbitrary arguments through the
27 Android application runtime. It is, however, possible to affect the
28 VM behavior through certain system properties.
30 <p>For all of the features described below, you would set the system property
31 with <code>setprop</code>,
32 issuing a shell command on the device like this:
33 <pre>adb shell setprop <name> <value></pre>
35 <p><strong>The Android runtime must be restarted before the changes will take
36 effect</strong> (<code>adb shell stop; adb shell start</code>). This is because the
37 settings are processed in the "zygote" process, which starts early and stays
40 <p>You may not be able to set <code>dalvik.*</code> properties or restart
41 the system as an unprivileged user. You can use
42 <code>adb root</code> or run the <code>su</code> command from the device
43 shell on "userdebug" builds to become root first. When in doubt,
44 <pre>adb shell getprop <name></pre>
45 will tell you if the <code>setprop</code> took.
47 <p>If you don't want the property to evaporate when the device reboots,
48 add a line to <code>/data/local.prop</code> that looks like:
49 <pre><name> = <value></pre>
51 <p>Such changes will survive reboots, but will be lost if the data
52 partition is wiped. (Hint: create a <code>local.prop</code>
53 on your workstation, then <code>adb push local.prop /data</code>. Or,
55 <code>adb shell "echo name = value >> /data/local.prop"</code> -- note
56 the quotes are important.)
59 <h2><a name="checkjni">Extended JNI Checks</a></h2>
61 <p>JNI, the Java Native Interface, provides a way for code written in the
62 Java programming language
63 interact with native (C/C++) code. The extended JNI checks will cause
64 the system to run more slowly, but they can spot a variety of nasty bugs
65 before they have a chance to cause problems.
67 <p>There are two system properties that affect this feature, which is
68 enabled with the <code>-Xcheck:jni</code> command-line argument. The
69 first is <code>ro.kernel.android.checkjni</code>. This is set by the
70 Android build system for development builds. (It may also be set by
71 the Android emulator unless the <code>-nojni</code> flag is provided on the
72 emulator command line.) Because this is an "ro." property, the value cannot
73 be changed once the device has started.
75 <p>To allow toggling of the CheckJNI flag, a second
76 property, <code>dalvik.vm.checkjni</code>, is also checked. The value
77 of this overrides the value from <code>ro.kernel.android.checkjni</code>.
79 <p>If neither property is defined, or <code>dalvik.vm.checkjni</code>
80 is set to <code>false</code>, the <code>-Xcheck:jni</code> flag is
81 not passed in, and JNI checks will be disabled.
83 <p>To enable JNI checking:
84 <pre>adb shell setprop dalvik.vm.checkjni true</pre>
86 <p>You can also pass JNI-checking options into the VM through a system
87 property. The value set for <code>dalvik.vm.jniopts</code> will
88 be passed in as the <code>-Xjniopts</code> argument. For example:
89 <pre>adb shell setprop dalvik.vm.jniopts forcecopy</pre>
91 <p>For more information about JNI checks, see
92 <a href="jni-tips.html">JNI Tips</a>.
95 <h2><a name="assertions">Assertions</a></h2>
97 <p>Dalvik VM supports the Java programming language "assert" statement.
98 By default they are off, but the <code>dalvik.vm.enableassertions</code>
99 property provides a way to set the value for a <code>-ea</code> argument.
101 <p>The argument behaves the same as it does in other desktop VMs. You
102 can provide a class name, a package name (followed by "..."), or the
105 <p>For example, this:
106 <pre>adb shell setprop dalvik.vm.enableassertions all</pre>
107 enables assertions in all non-system classes.
109 <p>The system property is much more limited than the full command line.
110 It is not possible to specify more than one <code>-ea</code> entry, and there
111 is no way to specify a <code>-da</code> entry. There is presently no
112 equivalent for <code>-esa</code>/<code>-dsa</code>.
115 <h2><a name="verifier">Bytecode Verification and Optimization</a></h2>
117 <p>The system tries to pre-verify all classes in a DEX file to reduce
118 class load overhead, and performs a series of optimizations to improve
119 runtime performance. Both of these are done by the <code>dexopt</code>
120 command, either in the build system or by the installer. On a development
121 device, <code>dexopt</code> may be run the first time a DEX file is used
122 and whenever it or one of its dependencies is updated ("just-in-time"
123 optimization and verification).
125 <p>There are two command-line flags that control the just-in-time
126 verification and optimization,
127 <code>-Xverify</code> and <code>-Xdexopt</code>. The Android framework
128 configures these based on the <code>dalvik.vm.dexopt-flags</code>
132 <pre>adb shell setprop dalvik.vm.dexopt-flags v=a,o=v</pre>
133 then the framework will pass <code>-Xverify:all -Xdexopt:verified</code>
134 to the VM. This enables verification, and only optimizes classes that
135 successfully verified. This is the safest setting, and is the default.
136 <p>You could also set <code>dalvik.vm.dexopt-flags</code> to <code>v=n</code>
137 to have the framework pass <code>-Xverify:none -Xdexopt:verified</code>
138 to disable verification. (We could pass in <code>-Xdexopt:all</code> to
139 allow optimization, but that wouldn't necessarily optimize more of the
140 code, since classes that fail verification may well be skipped by the
141 optimizer for the same reasons.) Classes will not be verified by
142 <code>dexopt</code>, and unverified code will be loaded and executed.
144 <p>Enabling verification will make the <code>dexopt</code> command
145 take significantly longer, because the verification process is fairly slow.
146 Once the verified and optimized DEX files have been prepared, verification
147 incurs no additional overhead except when loading classes that failed
150 <p>If your DEX files are processed with verification disabled, and you
151 later turn the verifier on, application loading will be noticeably
152 slower (perhaps 40% or more) as classes are verified on first use.
154 <p>For best results you should force a re-dexopt of all DEX files when
155 this property changes. You can do this with:
156 <pre>adb shell "rm /data/dalvik-cache/*"</pre>
157 This removes the cached versions of the DEX files. Remember to
158 stop and restart the runtime (<code>adb shell stop; adb shell start</code>).
160 <p>(Previous version of the runtime supported the boolean
161 <code>dalvik.vm.verify-bytecode</code> property, but that has been
162 superceded by <code>dalvik.vm.dexopt-flags</code>.)</p>
165 <h2><a name="execmode">Execution Mode</a></h2>
167 <p>The current implementation of the Dalvik VM includes three distinct
168 interpreter cores. These are referred to as "fast", "portable", and
169 "debug". The "fast" interpreter is optimized for the current
170 platform, and might consist of hand-optimized assembly routines. In
171 constrast, the "portable" interpreter is written in C and expected to
172 run on a broad range of platforms. The "debug" interpreter is a variant
173 of "portable" that includes support for profiling and single-stepping.
175 <p>The VM may also support just-in-time compilation. While not strictly
176 a different interpreter, the JIT compiler may be enabled or disabled
177 with the same flag. (Check the output of <code>dalvikvm -help</code> to
178 see if JIT compilation is enabled in your VM.)
180 <p>The VM allows you to choose between "fast", "portable", and "jit" with an
181 extended form of the <code>-Xint</code> argument. The value of this
182 argument can be set through the <code>dalvik.vm.execution-mode</code>
185 <p>To select the "portable" interpreter, you would use:
186 <pre>adb shell setprop dalvik.vm.execution-mode int:portable</pre>
187 If the property is not specified, the most appropriate interpreter
188 will be selected automatically. At some point this mechanism may allow
189 selection of other modes, such as JIT compilation.
191 <p>Not all platforms have an optimized implementation. In such cases,
192 the "fast" interpreter is generated as a series of C stubs, and the
193 result will be slower than the
194 "portable" version. (When we have optimized versions for all popular
195 architectures the naming convention will be more accurate.)
197 <p>If profiling is enabled or a debugger is attached, the VM
198 switches to the "debug" interpreter. When profiling ends or the debugger
199 disconnects, the original interpreter is resumed. (The "debug" interpreter
200 is substantially slower, something to keep in mind when evaluating
203 <p>The JIT compiler can be disabled on a per-application basis by adding
204 <code>android:vmSafeMode="true"</code> in the <code>application</code>
205 tag in <code>AndroidManifest.xml</code>. This can be useful if you
206 suspect that JIT compilation is causing your application to behave
210 <h2><a name="dp">Deadlock Prediction</a></h2>
212 <p>If the VM is built with <code>WITH_DEADLOCK_PREDICTION</code>, the deadlock
213 predictor can be enabled with the <code>-Xdeadlockpredict</code> argument.
214 (The output from <code>dalvikvm -help</code> will tell you if the VM was
215 built appropriately -- look for <code>deadlock_prediction</code> on the
216 <code>Configured with:</code> line.)
217 This feature tells the VM to keep track of the order in which object
218 monitor locks are acquired. If the program attempts to acquire a set
219 of locks in a different order from what was seen earlier, the VM logs
220 a warning and optionally throws an exception.
222 <p>The command-line argument is set based on the
223 <code>dalvik.vm.deadlock-predict</code> property. Valid values are
224 <code>off</code> to disable it (default), <code>warn</code> to log the
225 problem but continue executing, <code>err</code> to cause a
226 <code>dalvik.system.PotentialDeadlockError</code> to be thrown from the
227 <code>monitor-enter</code> instruction, and <code>abort</code> to have
230 <p>You will usually want to use:
231 <pre>adb shell setprop dalvik.vm.deadlock-predict err</pre>
232 unless you are keeping an eye on the logs as they scroll by.
234 <p>Please note that this feature is deadlock prediction, not deadlock
235 detection -- in the current implementation, the computations are performed
236 after the lock is acquired (this simplifies the code, reducing the
237 overhead added to every mutex operation). You can spot a deadlock in a
238 hung process by sending a <code>kill -3</code> and examining the stack
239 trace written to the log.
241 <p>This only takes monitors into account. Native mutexes and other resources
242 can also be the cause of deadlocks, but will not be detected by this.
245 <h2><a name="stackdump">Stack Dumps</a></h2>
247 <p>Like other desktop VMs, when the Dalvik VM receives a SIGQUIT
248 (Ctrl-\ or <code>kill -3</code>), it dumps stack traces for all threads.
249 By default this goes to the Android log, but it can also be written to a file.
251 <p>The <code>dalvik.vm.stack-trace-file</code> property allows you to
252 specify the name of the file where the thread stack traces will be written.
253 The file will be created (world writable) if it doesn't exist, and the
254 new information will be appended to the end of the file. The filename
255 is passed into the VM via the <code>-Xstacktracefile</code> argument.
258 <pre>adb shell setprop dalvik.vm.stack-trace-file /tmp/stack-traces.txt</pre>
260 <p>If the property is not defined, the VM will write the stack traces to
261 the Android log when the signal arrives.
264 <h2><a name="dexcheck">DEX File Checksums</a></h2>
266 <p>For performance reasons, the checksum on "optimized" DEX files is
267 ignored. This is usually safe, because the files are generated on the
268 device, and have access permissions that prevent modification.
270 <p>If the storage on a device becomes unreliable, however, data corruption
271 can occur. This usually manifests itself as a repeatable virtual machine
272 crash. To speed diagnosis of such failures, the VM provides the
273 <code>-Xcheckdexsum</code> argument. When set, the checksums on all DEX
274 files are verified before the contents are used.
276 <p>The application framework will provide this argument during VM
277 creation if the <code>dalvik.vm.check-dex-sum</code> property is enabled.
279 <p>To enable extended DEX checksum verification:
280 <pre>adb shell setprop dalvik.vm.check-dex-sum true</pre>
282 <p>Incorrect checksums will prevent the DEX data from being used, and will
283 cause errors to be written to the log file. If a device has a history of
284 problems it may be useful to add the property to
285 <code>/data/local.prop</code>.
287 <p>Note also that the
288 <code>dexdump</code> tool always verifies DEX checksums, and can be used
289 to check for corruption in a large set of files.
292 <h2><a name="general">General Flags</a></h2>
294 <p>In the "Honeycomb" release, a general mechanism for passing flags to
295 the VM was introduced:
297 <pre>adb shell setprop dalvik.vm.extra-opts "flag1 flag2 ... flagN"</pre>
299 <p>The flags are separated by spaces. You can specify as many as you want
300 so long as they all fit within the system property value length limit
301 (currently 92 characters).
303 <p>The extra-opts flags will be added at the end of the command line,
304 which means they will override earlier settings. This can be used, for
305 example, to experiment with different values for <code>-Xmx</code> even
306 though the Android framework is setting it explicitly.
308 <address>Copyright © 2008 The Android Open Source Project</address>